1. Field of the Invention
The present invention generally relates to information storage devices, and, more particularly, to an information storage device having a uniaxial tracking mechanism as a pickup.
When performing a pull-in operation to move from a seek control operation to a track following control operation in an optical disk device, the velocity of the objective lens is measured to determine a deceleration current. The deceleration current is then outputted so that the velocity is lowered enough to perform a stable track pull-in operation. After that, the operation is switched to the track following control operation.
Meanwhile, to lower the cost of the device, a digital servo system using MPU or DSP is used for controlling a beam-spot tracking mechanism, and a uniaxial system is employed for the tracking mechanism.
In view of this, it is necessary to develop a control method suitable for sampling operations, and also, it is necessary to achieve steadier track pull-in operations.
2. Description of the Related Art
As mentioned above, in a pull-in operation to move from a seek control operation to a track following control operation in an optical disk device, the velocity of the objective lens is measured to determine a deceleration current. The deceleration current is then outputted so that the velocity is lowered enough to perform a stable track pull-in operation. After that, the operation is switched to the track following control operation.
To perform a stable track pull-in operation, the pulse height of a deceleration pulse, i.e., an acceleration á, and the pulse width, i.e., a time t, can be determined using the following equations:t=2d/v  (1)á=v2/2d  (2)
wherein d is the remaining distance to a target track, and v is the velocity at the time of pull-in start. Japanese Laid-Open Patent Application No. 3-37876 discloses this method in detail.
In recent years, a tracking control operation for an optical disk device has been performed more and more often by a digital arithmetic unit, such as DSP (Digital Signal Processor), to lower the cost. In such a case, control output is carried out in predetermined sampling cycles. Even if the pulse width t is determined from the velocity v at the pull-in starting time, the resolution is determined by the control sampling cycles of the DSP.
For instance, in a case where a deceleration pulse is outputted at v=8.3 mm/s and at a point half a track before a target track in a 1.1-μm track-pitch optical disk medium, the deceleration pulse height, i.e., the acceleration á, can be determined from the equation (2) as:á=−62.6 [m/s2]
The deceleration pulse width, i.e., the time t, can be determined from the equation (1) as:t=132.5 μs
If sampling is performed at a frequency of 60 kHz, the time t is equivalent to 7.95 cycles. Accordingly, a 7-cycle deceleration pulse is outputted.
Since 7 cycles are equivalent to the time t=116.7 μs, only a deceleration velocity v=át=7.3 mm/s is obtained. For an intended deceleration velocity v of 8.3 mm/s, a residual velocity of 1.0 mm/s is caused. This residual velocity adversely affects the stability in the track pull-in operation.
In an actual device, the velocity v is determined by measuring the cycle T of a tracking error signal and dividing the track pitch p by the cycle T. Accordingly, the velocity v can be expressed as:v=p/T  (3)
From the equation (3), the time t can be expressed as:T=2dT/p  (4)
From the equation (4), the acceleration á can be expressed as:á=p2/(2dT2)  (5)
If noise exists in a tracking error signal, an error is caused in a measured value of the cycle T. The error of the cycle T affects the pulse width t of the deceleration pulse based on the equation (4) and also the acceleration á based on the equation (5). Here, the acceleration á is in inverse proportion to the square of T. For this reason, the error greatly affects the acceleration á, and hinders accurate control operations. Conventionally, a decelerating operation is carried out by a single deceleration pulse having the pulse width t and the pulse height a determined by the equations (4) and (5).
A suitable point to detect the present position of a beam spot in the vicinity of a target track in a seek control operation is half a track before the target track. In a case where the medium has a 1.1-μm pitch, for instance, the suitable beam-spot detecting position is 0.55 μm before a target track.
In a case where the velocity v at the starting time of a track pull-in operation is 8.0 mm/s, to reduce the velocity v to 0 mm/s while moving half a track, a deceleration pulse having a pulse width of 137.5 μs and a pulse height of 58.2 m/s2 is required. In a uniaxial tracking mechanism, the acceleration ability is low, and it is extremely difficult to obtain such a high acceleration. To lower the acceleration, the velocity v at the starting time of a track pull-in operation must be lowered. However, if the velocity is too low, the seek velocity control becomes unstable.
There is a method in which a remaining distance d to a target track is made longer so as to maintain allow acceleration, i.e., a pull-in deceleration pulse is outputted one track or 1.5 tracks before the target track. For instance, Japanese Laid-Open Patent Application No. 9-81940 discloses a method in which deceleration is started one track before a target track. In such a method, however, the deceleration pulse width t is larger, and if there is an error in the acceleration mechanism of the tracking actuator or if turbulence is externally caused during a decelerating operation, any of those changes cannot be accommodated. To solve such a problem, there is a method in which a deceleration pulse having a smaller width t is employed. However, this method also has a problem that the residual velocity at the starting time of a deceleration pulse cannot be made high enough to perform stable seek operations. Japanese Laid-Open Patent Application No. 9-102135 suggests a method in which a deceleration pulse is outputted one track before a target track, and the height of the deceleration pulse is then corrected half a track before the target track. In this method, however, an accurate velocity cannot be detected from a tracking error signal half a track before a target track, because deceleration is caused by the deceleration pulse. In Japanese Laid-Open Patent Application No. 9-102135, for instance, the track pitch is divided by the zero-cross cycle of a tracking error signal to obtain a velocity detection value VDET. However, since the obtained VDET is the mean velocity between the zero-cross cycles, the instantaneous velocity at the detection point of the latest zero-cross cannot be accurately measured when acceleration is caused.
As described so far, the problems with the prior art are that the deceleration of the beam spot cannot be made high enough after a seek velocity control operation, and that the beam spot velocity used for pulse height correction cannot be accurately measured. Also, with a pulse width and pulse height determined by the equations (4) and (5), the resolution of the pulse width deteriorates due to the sampling, and the height of a deceleration pulse, i.e., the high acceleration á, results in unstable track pull-in operations.